Radio-frequency identification (RFID) applications continue to increase in number and new uses of the technology have prompted innovative tag designs. Achieving smaller tag footprints is of particular importance, motivating the development of three-dimensional designs that make more efficient use of the antenna volume. However, most commercial tag designs are optimized either for on-metal or off-metal conditions, and their performance is greatly diminished when the surrounding environment differs from the intended one. For example, dipole antennas can be used for off-metal systems, but often fail to operate properly when mounted horizontally over or near metallic objects. Patch antennas and folded dipoles, on the other hand, have been implemented for on-metal systems, but generally have a low off-metal read range.
From the above discussion, it can be appreciated that it would be desirable to have an RFID tag, and a tag antenna, that can be used in both on- and off-metal applications.